Anne-Marie Nybo Andersen, senior researcher at the University of Copenhagen, said: "The main message from the paper is that miscarriages are a subject for prevention."

Ms Nybo Andersen said the paper was significant as it showed the relative importance of different lifestyle factors in causing miscarriage, rather than more specific factors, such as certain pharmaceutical drugs.

As the findings were from the health perspective of a population, they could apply to lots of people - from individual couples to people in charge of maternity policies, work place regulations and supporting students who get pregnant, she said.

She added: "Everybody, young men and women, as well as those who have political responsibilities should bear in mind that postponing pregnancy to the mid-30s implies a seriously increased risk of miscarriage."

Healthy-pregnancy advice

Pregnancies included in the Danish National Birth Cohort between 1996 and 2002 were analysed for the study. Researchers found 3.5% of the women miscarried.

They looked for links between the miscarriages and lifestyle by collecting data through computer-assisted telephone interviews.

Caroline Overton, spokesperson at the Royal College of Obstetricians and Gynaecologists, said: "This is a very interesting study in terms of the very large population size."

She said it confirmed much of the advice currently given out in the UK to promote healthy pregnancy, such as not drinking, but questioned how applicable the study would be to British women.

On the subject of the findings, Ms Overton added: "Twenty kilograms is the size of a large holiday suitcase. Most of us would only lift that very occasionally."

Women wanting to conceive should also eat a balanced diet, make sure they are not "too skinny", or overweight, cut out smoking and ask their partners to follow suit, she said.